1. Field of the Invention
The present invention relates to apparatuses and methods for controlling an engine favorably mountable on a vehicle and in particular to an apparatus and a method for controlling an engine wherein spark ignition burning mode and homogeneous charge compression ignition burning mode can be selectively established as burning mode.
2. Related Art
As a combustion system for achieving both the enhancement of fuel efficiency performance and the enhancement of exhaust performance (exhaust emission characteristic) in engines (internal combustion engines) used in automobiles and the like, homogeneous charge compression ignition combustion system has received attention. A homogeneous charge compression ignition combustion system compresses an air-fuel mixture composed of fuel (gasoline) and air to cause homogeneous charge compression ignition combustion. In the homogeneous charge compression ignition combustion system, the following can be implemented as compared with the spark ignition combustion system in which an air-fuel mixture is ignited and burned by a spark produced by a spark plug: the enhancement of efficiency by a high compression ratio, the reduction of pump loss, and the reduction of cooling loss by fast burn. Therefore, it is possible to reduce fuel consumption and to reduce the concentration of NOx in exhaust gas by the low-temperature combustion of an air-fuel mixture. For this reason, it is possible to achieve both the enhancement of fuel efficiency performance and the enhancement of exhaust performance.
One of means for implementing homogeneous charge compression ignition combustion is the introduction of EGR. In spark ignition combustion, the air-fuel ratio is relatively rich and it can be carried out in a range in which the EGR rate is relatively low. In homogeneous charge compression ignition combustion to which the above EGR is applied, the air-fuel ratio is relatively lean and it can be carried out in a range in which the internal EGR rate is relatively high. There is a combustion instability range in which both types of combustion are instable between these ranges. The following methods are used to introduce EGR: a method of providing a negative overlap period, which is a period during which both an intake valve and an exhaust valve are closed in the exhaust strokes and thereby leaving combustion exhaust gas (exhaust gas) in a cylinder (combustion chamber) (internal EGR); a method of providing a bypass from an exhaust passage upstream of an intake valve and thereby sucking exhaust gas together with fresh air (external EGR); and a method of opening an exhaust valve in the intake stroke and thereby sucking exhaust gas again (exhaust re-sucking).
When the above homogeneous charge compression ignition combustion system is adopted, a large quantity of exhaust gas is introduced into a combustion chamber and this limits the quantity of fresh air. For this reason, usually, the homogeneous charge compression ignition combustion system is applicable only to the low load side where request engine torque is small. Since it is required to ensure a finite length of time for the chemical reaction of fuel in the strokes of intake, compression, expansion, and exhaust, it is applicable only to the low engine speed side.
For this reason, it is usually required to take the following measure to attain a driver's requested engine torque when a homogeneous charge compression ignition combustion system is applied to an automobile engine: in the low-load, low-rotation range, the engine is operated in homogeneous charge compression ignition burning mode; in the other ranges, the engine is operated in spark ignition burning mode; and in operating range change, the burning mode is changed from homogeneous charge compression ignition burning mode to spark ignition burning mode or from spark ignition burning mode to homogeneous charge compression ignition burning mode.
When the burning mode is changed between spark ignition burning mode and homogeneous charge compression ignition burning mode as mentioned above, engine parameters governing combustion are changed. The engine parameters include: fuel injection mode (the quantity of fuel injected from a fuel injection valve, fuel injection starting time, the number of times of injection in one burning cycle, and the like); firing mode (ignition timing with which a spark is discharged from a spark plug and the like); the opening of a throttle valve for adjusting the quantity of air flowing into a combustion chamber; the lift amount and the valve opening period (a period from when a valve starts to open to when the valve finishes closing) of an intake valve for adjusting the quantity of air flowing into a combustion chamber or an exhaust valve; and the like. Since an actuator that controls engine parameters has a limited response characteristic, the following problem arises: in transition for burning mode change (during mode change), it is brought into the combustion instability range and misfire or torque fluctuation is caused.
As one of measures for solving this problem, the following method is known: in transition for change between spark ignition burning mode and homogeneous charge compression ignition burning mode, a transitional period is set to cope with the combustion instability range. (Refer to Patent Document 1, for example.) In the technology disclosed in Patent Document 1, homogeneous charge compression ignition combustion is carried out by the above-mentioned exhaust re-sucking. The technology involves a variable valve system capable of instantaneously changing the lift amounts of an intake valve and an exhaust valve for carrying out both homogeneous charge compression ignition combustion and spark ignition combustion. The response of air quantity to change in the opening of a throttle valve is slower than the response of gas in a combustion chamber with a valve operating mechanism. Therefore, a TSI range is provided between a homogeneous charge compression ignition combustion range and a spark ignition combustion range. In the TSI range, the lift amount of an intake valve is set to that for homogeneous charge compression ignition burning mode and a throttle valve is widely opened to carry out spark ignition combustion. Combustion is stabilized and torque fluctuation during mode change is reduced by providing the TSI range.    [Patent Document 1] Japanese Unexamined Patent Publication No. 2003-148180 (Japanese Patent No. 3936901)